Hi Hitesh, Please look at the "Writing the Subscriber Node" section here: http://www.ros.org/wiki/rospy_tutorials/Tutorials/WritingPublisherSubscriber#Writing_the_Subscriber_Node Your subscriber has no callback. The callback is what receives the message instance. Hope this helps, Ken On Sun, Apr 18, 2010 at 12:51 PM, hitesh dhiman wrote: > Hi Ken, > Actually i started out by following the tutorials. > Here's the code I'm using: > > velocity = rospy.Subscriber("cmd_vel", Twist) > > I checked using rxgraph, my node is subscribing to cmd_vel. I also used > rostopic echo /cmd_vel, i can see the velocities being published. Now the > problem is, how to extract the information from the message. > > If i try to type : > > print velocity.linear.x > > I get an error: velocity has no attribute x > > Even if i just do velocity.linear[0], or simply velocity.linear, i get the > error: > > velocity has no attribute linear > > The cmd_vel topic has a type Twist message, that means two cpp vectors, each > for linear and angular velocities. I'm not able to get the correct format to > access these vectors. > > On Mon, Apr 19, 2010 at 3:00 AM, Ken Conley wrote: >> >> Hi Hitesh, >> >> If you follow the rospy tutorials for publishing and subscribing [1] >> you will be able to subscribe to any type of ROS topic, including >> cmd_vel, and extract the fields that you want. >> >> [1] http://www.ros.org/wiki/rospy/Tutorials >> >>  - Ken >> >> On Sun, Apr 18, 2010 at 11:55 AM, hitesh dhiman >> wrote: >> > Hi ken, >> > Well I think you're correct. If I just do a print vel, it says >> > subscriber >> > object at 'address'...Basically what I'm doing is getting the cmd_vel >> > published from navigation stack. >> > On this note, is it possible to find out the correct way to get the >> > linear >> > and angular velocities from cmd_vel ? >> > >> > On Apr 19, 2010 2:34 AM, "Ken Conley" wrote: >> > >> > Hi hitesh, >> > >> > It would help if you cut and paste the exact error message and include a >> > sample program. I suspect that the program is using the subscriber >> > object >> > instead of the message object, but this is hard to tell without more >> > information. >> > >> > Ken >> > >> >> > On Apr 18, 2010 9:43 AM, "hitesh dhiman" >> >> > >> >> > wrote: > > Hi all, > I'... >> > >> > >> > ------------------------------------------------------------------------------ >> > Download Intel® Parallel Studio Eval >> > Try the new software tools for yourself. Speed compiling, find bugs >> > proactively, and fine-tune applications for parallel performance. >> > See why Intel Parallel Studio got high marks during beta. >> > http://p.sf.net/sfu/intel-sw-dev >> > _______________________________________________ >> > ros-users mailing list >> > ros-users@lists.sourceforge.net >> > https://lists.sourceforge.net/lists/listinfo/ros-users >> > >> > _______________________________________________ >> > ros-users mailing list >> > ros-users@code.ros.org >> > https://code.ros.org/mailman/listinfo/ros-users >> > >> > >> > _______________________________________________ >> > ros-users mailing list >> > ros-users@code.ros.org >> > https://code.ros.org/mailman/listinfo/ros-users >> > >> > >> > _______________________________________________ >> > ros-users mailing list >> > ros-users@code.ros.org >> > https://code.ros.org/mailman/listinfo/ros-users >> > >> > >> _______________________________________________ >> ros-users mailing list >> ros-users@code.ros.org >> https://code.ros.org/mailman/listinfo/ros-users > > > > -- > Regards, > Hitesh Dhiman > Electrical Engineering > National University of Singapore > > _______________________________________________ > ros-users mailing list > ros-users@code.ros.org > https://code.ros.org/mailman/listinfo/ros-users > >